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r;vor upadhAyA dIrgha:: ikaH

र्वोरुपधाया दीर्घ‌ इकः ONPANINI 82076

Lengthen nexttolast i इ u उ (of a root) before (padafinal) r र् v व्.

This affects onevowel true r bases like **gir- गिर् f "speech", **pur- पुर् f "city".

Their r र् is padafinal (A) when their /sup gets deleted —

**gir- गिर् + /su
→ girs गिर्स् by halGyAbbhyodIrgh...
→ !**gIr गीर् by this rule

**pur- पुर् + /su → !**pUr पूर् "city"

dhur धुर्+ + /su → dhUr धूर् "load"

and (B) when they become /pada by svAdiSvasarv... —

**pur- पुर् + /sup'
→ pUr पूर् + su सु by this rule
(kharavasAn... debarred by rossupi )
→ pUr पूर् + Su षु with /Satvam

→ !**pUrSu पूर्षु "in cities"

**gir- गिर् + /sup' → !**gIrSu गीर्षु, same steps

Why do we say padafinal?

No lengthening before vowel endings —

**gir- गिर् + /au → !**girau गिरौ

**gir- गिर् + /jas OR /zas → !**giras गिरस्

Why do we say "of a root"?

Tricky this one. Afaik, all onevowel true r bases count as roots, like those above. And catur चतुर् is not a root. So —

**catur- चतुर् + /sup'
→ !**caturSu चतुर्षु

Again afaik, bases ending in is इस् us उस्, like **barhis- बर्हिस्, are all non-roots and unaffected by this sUtra, with the only exception of **Azis- आशिस्, which is a root because it comes from /AG + zAs शास्. So when its s स् is padafinal, it becomes /ru by sasaju ससजु and triggers this rule —

Azis आशिस् + /su
→ Azis आशिस् by halGyAbbhyodIrgh...
→ Azir आशिर् by <10 sasajuSoruH
→ !**AzIr आशीर् by this rule

Azis आशिस् + /bhis
→ Azirbhis आशिर्भिस् by <10 sasajuSoruH
→ !**AzIrbhis आशीर्भिस् by this rule

Of course the /ru won't trigger rossupi, so kharavasAn... works as usual here —

Azis आशिस् + /sup'
→ Azir आशिर् + su सु by <10 sasajuSoruH
→ AzIr आशीर् + su सु by this rule
→ AzIH आशीः + su सु by kharavasAn...
→ AzIssu आशीःसु or AzIHsu by vAzari
→ !**AzISSu आशीःषु or AzIHsu with /Satvam